The present invention generally relates to methods of communicating Voice band Data over packet and low data rate digital networks, and more specifically relates to a method for detection and bypass of encoding/decoding of Voice band Data, in a telecommunication network consisting of more than one pair, or “tandem” of Voice band Data relay gateways, in order to reduce the end-to-end processing delay of the Voice band Data signal, therefore increasing the performance of the Voice band Data relay. In short, the present invention relates to a method in detection and avoidance of Voice band Data relay connection.
Most protocols have built-in timing requirements that cause the transmission to be terminated (or retried) if the time delay exceeds certain predefined threshold values. One protocol which presently exists is the V.42 LAPM protocol, and the timing requirement is referred to as ITU-T V.42, T400, wherein T400 is the detection phase timer, which governs the amount of the time that a control function in an originating or answering DCE waits for the ADP (answer detection pattern) or the ODP (originator detection pattern), respectively. The default value for the timer T400 is 750 ms. This is the estimated maximum propagation delay of all required transmissions at the data rate used in any V-Series modem that uses asynchronous-to-synchronous conversion.
Voice band Data Relay is used to allow Voice band Data transmission between Voice band Data terminals where, in addition to the PSTN or ISDN, a portion of the transmission path utilizes Low Rate Digital Networks or a packet network. Voice band Data relay is a technique by which a calling Voice band Data terminal supporting traditional V Series modem procedure is connected to a gateway through a Low Rate Digital Network or a packet network to another gateway which makes a PSTN call to the called Voice band Data equipment. Once the PSTN calls are established on both ends, the two Voice band Data terminals are virtually linked.
An end-to-end circuit connecting Voice band Data terminals may consist of multiple pairs of Voice band Data relay gateways. Each gateway adds delays to the overall processing of the signaling between two Voice band Data terminals in real-time. Therefore, when a network consists of two or three pairs or tandems of Voice band Data relays, the delay associated with demodulation/re-modulation of the Voice band Data signal and the associated protocol can result in Voice band Data transmission failure, due to the expiration of the timer at the originating or the answering side.